The purpose of this project has been to develop and characterize an in vivo transgenic mouse model for examining mechanisms of microbial induced immune activation of HIV-1 as well as for testing of candidate treatments that might block this process. Progress was achieved this year in the following areas. Continued studies on the cellular source of HIV-1 expression in spleen indicated that in addition to macrophages and dendritic cells (DC), B lymphocytes can also serve as a source of virus following microbial stimulation. A project was initiated to characterize the receptors and signalling pathways responsible for HIV induction in these splenic APC populations. This work identified TLR 2 and 4 binding microbial ligands, IL-1, IL-18, GM-CSF, CD40L and CpG oligonucleotides as agonists of HIV-expression and IL-4 and IFN alpha/beta as antagonists. CD40L stimulation was studied in detail and was shown to be largely responsible for the induction of virus from DC by polyclonally activated or Ag stimulated T lymphocytes through its interaction with CD40. Indeed, this T cell-APC interaction was shown to be a major pathway involved in the stimulation of HIV expression during experimental malaria infection. Thus, CD4 depletion or administration of a CD40L antagonist both dramatically inhibited virus production in acutely infected mice. In related work, prednisolone or an Env binding CD4-toxin conjugate, two agents previously shown by us to reduce HIV-1 expression in vitro both markedly inhibited in vivo virus production stimulated by M. avium. infection. The above observations formally establish the utility of the transgenic mouse model as a system for testing interventions that inhibit microbial induced immune activation of HIV-1 in vivo. In separate studies, it was shown that bone marrow derived DC from transgenic mice induce substantial levels of both HIV specific Ab as well as CTL activity against transfected targets when used to immunize either control FVB/N or transgenic mice. The latter observations establish a model system for developing DC based vaccination schemes against HIV.